1. Field of the Invention
This invention relates in general to offshore oil and gas well equipment and in particular to a coupling for a subsea riser to a platform or surface deck.
2. Description of the Prior Art
In offshore oil and gas wells, a pipe, often referred to as a riser, is used for communicating between the wellhead, located at the sea bottom, and the surface. The riser is supported at one end from a floating platform, or other surface structure, and is coupled at the other end to the subsea wellhead. Because of movement of the surface platform caused by wave motion and ocean currents, the riser must be coupled to the surface platform in a manner that allows for this movement while maintaining adequate tension on the riser. This is typically accomplished through the use of tensioners that couple between the platform and riser. The tensioner is usually in the form of a piston and cylinder containing pressurized fluid. The piston and cylinder are coupled to the riser by means of a tension ring. Most prior art tension rings are formed as a unitary ring which surrounds the riser.
The upper section of the riser is typically provided with helical threads for engagement with threads of the tension ring itself, or locking members or wedges used to retain the tension ring in place upon the riser. Those tension ring assemblies that do employ locking members, slips or wedges, besides adding to the number of parts that must be manufactured and machined, also require the tension ring to have a larger diameter for accommodating locking members.
It is often necessary to adjust the position of the tension ring along the riser. This is accomplished by rotating the threaded tension ring or the locking member assembly upon the threaded riser so that the tension ring can be moved to the desired position along the riser. Oftentimes, especially after extended periods of use without adjustment, the threads of the riser and tension ring assembly may tend to seize together making it difficult to rotate the threaded tension ring or locking member assembly. Damage and deformation of exposed threads of the riser may also make rotation difficult as the tension ring assembly is moved along the riser.
What is therefore needed is an improved riser tensioner and tensioning ring that is simple in design, is easily constructed and that can be positioned or repositioned on the riser and which does not require the need for locking members or slips that increase the required diameter of the tension ring or necessitate the manufacturing of additional components.